Parathyroid hormone (PTH) strongly regulates cellular differentiation and function in bone and cartilage. The amino-terminus of intact PTH(1-84) activates type-1 PTH/PTHrP receptors (PTH1Rs) to generate cAMP and other second messengers that control gene expression and cellular function in skeletal tissue. It now is clear that bone cells also express a distinct class of PTH receptors ("CPTHRs"). These recognize only the carboxyl(C)-terminal portion of intact PTH(1-84), a domain extensively conserved throughout evolution yet unable to bind or activate PTH1Rs. PTH C-fragments are secreted in a calcium-regulated manner by the parathyroid glands and generated also by peripheral cleavage of intact PTH. They comprise most of the PTH in blood and accumulate to high levels in renal failure, which is associated with growth retardation and osteodystrophy. The CPTH fragment hPTH(7-84), a model of extended CPTH fragments present in blood, exerts anticalcemic effects in vivo and antiresorptive effects in vitro that appear not to involve PTH1R activation. Clonal, PTH1R-null murine osteocytes, osteoblasts, marrow stromal cells and chondrocytes specifically bind the CPTHR radioligand 125I [Tyr34]hPTH(19-84) and show identical patterns of ligand selectivity for a series of truncated CPTH peptides. In skeletal cells in vitro, CPTH fragments can elicit increases in cytosolic Ca++, PKC and MAP kinase; control gene expression; regulate osteoclast formation and promote osteoblast and osteocyte apoptosis. This project will address the hypothesis that CPTHRs are expressed normally by bone cells and can mediate regulation of skeletal function by intact PTH and circulating CPTH fragments. CPTHR cDNA(s) will be cloned from an established osteocyte cDNA library, CPTHR function will be reconstituted by expressing these cDNAs in CPTHR-null host cells and tissue-specific CPTHR expression will be analyzed in mice by in situ hybridization and Northern analysis (Aim I). Cellular targets and actions involved in CPTHR regulation of bone resorption and osteoclast formation in vitro will be defined, as will CPTH ligand domains responsible for these actions (Aim II). Genes strongly regulated by CPTHRs in PTH1R-null clonal bone cells will be identified, and control of the expression of such genes by CPTH peptides in vivo will be sought (Aim III). This research will provide new information and reagents required to further define the roles of CPTHRs in normal bone physiology and, perhaps, in the etiology of skeletal diseases associated with disorders such as chronic renal failure.